Compound semiconductor devices have been widely used in communication and for display. Due to recent requirement of blue light sources, gallium nitride-based III-V Group compound semiconductor devices have become the center of research and development. Gallium nitride-based III-V Group compound semiconductor devices generally comprise InGaN, GaAlN, and InAlGaN. The most efficient substrate used to grow gallium nitride-based III-V Group compound semiconductor devices is Al.sub.2 O.sub.3 single-crystal substrate. However, because the Al.sub.2 O.sub.3 substrate can not conduct electricity, in producing a device such as a light-emitting diode (LED), its p-contact and n-contact must be fabricated on the same surface and connected to the p-type layer and the n-type layer of the LED via an ohmic contact layer, respectively. The contact is generally made of metallic material of a very thin thickness to prevent from shielding light. Therefore, the electronic characteristic of the ohmic contact layer has an enormous influence on the efficiency of the LED. Moreover, the hole mobility of the p-type layer of gallium nitride-based III-V Group compound semiconductor device is low, resulting in a high impedance of the p-type layer. Therefore, the diffusion of the hole current becomes an issue in the production of this device.
In U.S. Pat. No. 5,563,422, the Nichia company of Japan discloses a method of producing a gallium nitride-based III-V Group compound semiconductor device, wherein a metal film is electroplated on the p-type layer and the diffusion of the hole current is increased by annealing. Because hydrogen atoms bonded with the acceptors will be driven out during the annealing process, the acceptor impurities can be activated so as to improve the electronic characteristic of the ohmic contact layer. However, for metals such as nickel, chromium, aluminum, and gold on the p-type layer of gallium nitride, the efficiency and impedance of the ohmic contact can not be effectively improved.